Semiconductor device fabrication typically involves a number of processes that may be used to form various features and multiple levels or layers of semiconductor devices on a top surface of a semiconductor wafer or another appropriate substrate. For example, lithography is a process that typically involves transferring a prescribed circuit pattern to a resist layer formed on a top surface of the wafer. During the lithography process, the resist layer is formed on the top surface by dispensing a small amount of a liquid resist solution near the center of the wafer, distributing the resist solution across the top surface by a spin-coating process, exposing the resist to radiation in a prescribed pattern, and subsequently developing the pattern.
Resist spin coating is conducted in an automated track system using wafer handling equipment that transports the wafers between the various lithography operation stations, such as resist spin coating stations, developing stations, baking stations, and chilling stations. Automated wafer tracks enable various processing operations to be carried out simultaneously. One family of automated track systems widely used in the industry is the line of coater/developer tracks commercially available from Tokyo Electron Limited under the Clean Track brand.
During the spin-coating process, the wafer is held on a disk shaped, rotating spin chuck. The diameter of the spin chuck is slightly less than the diameter of the wafer. The spin chuck is positioned so that the wafer is oriented in a level horizontal plane. In operation, the spin chuck supports a backside of the wafer and applies suction to the wafer backside to hold the wafer in place as the chuck rotates. An amount of the liquid resist solution is applied at the center of the top surface of the rotating wafer. The spin chuck then rotates the wafer at a high rotational velocity to spread the liquid resist solution radially outward from the center of the wafer by centrifugal force towards the wafer's peripheral rim to coat the top surface.
Ideally, all excess liquid resist solution is ejected from the peripheral edge of the wafer. In practice, however, some excess liquid resist solution tends to collect along the peripheral edge of the wafer as an artifact of the spin-coating process. The accumulated liquid forms an edge bead as the resist solution solidifies. Portions of the edge bead may subsequently detach from the wafer and become a source of particulate contamination during subsequent process steps. Particulate contamination may unfortunately contribute to the yield loss of the integrated circuits being built on the wafer. Consequently, the peripheral edge of the wafer is typically cleaned in an edge bead removal process during, or after, the spin-coating process. The cleaning process removes the resist from an annular region of the layer near the peripheral edge and the edge bead carried by the annular region.
One conventional approach for removing the annular resist region and edge bead is to direct a stream of an edge bead removal chemical, such as a solvent, onto the wafer edge while the wafer is spinning. The solvent stream may be dispensed through a nozzle aimed toward the backside edge of the wafer, in which case the solvent flows about the peripheral edge to the top surface of the wafer. One shortcoming of this edge bead removal process is that the solvent flow and angular velocity of the spin chuck must be accurately controlled to cause the solvent to flow about the peripheral edge.
Another conventional approach for removing the annular resist region and edge bead is to dispense the solvent stream through a nozzle directly onto the top surface of the wafer. The nozzle is located at a significant height above the top surface. The solvent stream dissolves and removes a portion of the spin-on film including the edge bead. One shortcoming of this edge bead removal process is that, because of the physical separation, the solvent stream must be delivered with a high spatial accuracy. However, the delivery of the solvent stream is difficult to control because of factors such as variations in wafer size, the accuracy of centering the wafer in the spin chuck, and positioning errors in locating the nozzle used to spray the solvent stream. As a result, the radial width of the annular region of the resist removed by the chemical may be difficult to precisely control and is inherently unpredictable.
Conventional chemical edge bead removal processes suffer from several additional shortcomings. For example, conventional chemical edge bead removal processes require a process time of about 15 seconds to about 20 seconds per wafer and an angular velocity of 500 to 2000 revolutions per minute. Moreover, the amount of chemical used in conventional edge bead removal processes add to the volume of waste generated in the integrated circuit fabrication process. The annular width of the removed annular region of resist also reduces the usable area on the top surface of the wafer available for device fabrication. Conventional chemical edge bead removal processes have the ability to remove a 0.5 mm wide annular region with an accuracy of +0.5 mm, which represents a significant reduction in the useable wafer area. The relatively large annular width increases in its detrimental effect as devices are scaled to smaller critical dimensions and more densely packed.
What is needed, therefore, is a device for removing the resist edge bead that overcomes these and other deficiencies of conventional spin coaters and edge bead removal methods.